Recently, the speed of high-speed large-capacity communication using optical fiber has reached 10-40 Gbit/s, and further increase and expansion of the transmission speed and the transmission range are approaching their limits. The major cause of this “wavelength dispersion” (CD: Chromatic Dispersion) and “optical fiber non-linear effects.” Wavelength dispersion is a phenomenon in which lights of different wavelengths are transmitted at different speeds in optical fiber. The optical spectrum of an optical signal modulated at a high speed contains different wavelength components, and each wavelength component reaches a receiving end at a different time by the wavelength dispersion of the optical fiber used as a transmission line. Consequently, it is known that a light waveform after transmission suffers a large waveform distortion. Moreover, an optical fiber non-linear effect is a phenomenon in which while traveling in optical fiber, an optical signal induces extra phase modulation (frequency chirp) on itself or an optical signal advancing side by side by an intensity modulation component that the optical signal itself has. Still larger waveform distortion occurs by mutual interaction between this phase-modulated component and the above-mentioned wavelength dispersion.
Heretofore, in transmitting digital information by optical communication, there is widely used binary NRZ (Non-Return-to-Zero) modulation in which an electric digital signal as high as 2.5-40 GHz/s is inputted into an optical modulator or optical source, and an optical signal is turned on/off directly by it. However, since the NRZ signal is susceptible to the wavelength dispersion and the non-linear effect, various new modulation schemes are being studied for the purpose of reducing these influences in order to increase a transmission speed and to prolong a transmission range. As such modulation schemes, for example, A. Hirano, Y. Miyamoto, K. Yonenaga, A. Sano and H. Toba, “40 Gbit/s L-band transmission experiment using SPM-tolerant carrier-suppressed RZ format,” IEE ELECTRONICS LETTERS, 9 Dec. 1999, Vol. 35, No. 25 discloses the CSRZ (Carrier-Suppressed Return-to-Zero) modulation and K. Yonenaga and S. Kuwano, “Dispersion-Tolerant Optical Transmission Using Duobinary Transmitter and Binary Receiver,” JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 15, No. 8, AUGUST 1997, pp. 1530-1537 discloses the optical duobinary modulation. The former is a modulation scheme in which an optical signal is converted into a pulse and modulated so that adjacent pulses may have optical phases of zero or π alternately, whereby tolerance against the non-linear effect that poses a problem in long distance transmission is increased. The latter is a light modulation scheme that improves wavelength dispersion tolerance largely by changing optical phase by 180° when the power of an optical signal changes as a mark, a space, and a mark.
In modulation of an optical signal like these, the need for strictly controlling an optical waveform and an optical phase modulation quantity arises, and stabilization control of the bias-voltage of the optical modulator becomes extremely important. As a conventional example, for example, JP-A No. 2003-283432 discloses this control. FIG. 4 is a block diagram of a CSRZ optical transmitter using conventional stabilization control of the bias-voltage like this. In this example, signal light outputted from a laser optical source 101 proceeding along a path 112 is first converted to a CSRZ pulse of 20 GHz by a conventional optical modulation device 114, then subjected to intensity modulation by a 20-Gbit/s NRZ data signal that is an electrical signal inputted from a data signal input terminal 124, and outputted as a 20-Gbit/s CSRZ optical waveform. As an optical modulator 102, it is common to use the Mach-Zehnder (MZ) type optical modulator based on a waveguide substrate of a material, such as Lithium Niobate (LN). A light input path 103 and a light output path 104 are provided in front of and behind the optical modulator 102. The optical modulator 102 is of a dual-drive type such that mutually phase reversed 10-GHz clock signals are impressed on two traveling wave electrode input terminals 107-1, 107-2, respectively. In the middle of the traveling wave electrode input terminal 107-2, a bias control signal input terminal 111 that allows a low-speed (direct current to about a few MHz at the highest) bias-voltage Vb to be applied through a bias tee 110. The bias-voltage Vb is used in order to set up an operating point of the optical modulator as will be described later.
FIG. 5 shows a principle of generating the CSRZ optical pulse with the dual-drive MZ type optical modulator. FIG. 5B is optical transmission characteristics of the dual-drive MZ modulator with a vertical axis representing optical transmittance and a horizontal axis representing a difference voltage applied to the two electrodes. The two parameters hold a sinusoidal relationship. A voltage width corresponding to a single period thereof is denoted as V2π. To generate the CSRZ optical pulse, sinusoidal-waveform clock voltages (frequency: 10 GHz) whose phases are mutually reversed are applied to the two electrodes. A difference voltage across the two electrodes also becomes a sinusoidal wave, as shown in the figure. Hereinafter the voltage amplitude will be called simply “clock amplitude.” The difference voltage is set up so that this clock amplitude may be about V2π and the bias-voltage Vb that centers in it may coincide with a bottom of the sinusoidal wave (optimal point) where the optical transmittance is minimized. When the bias-voltage coincides with the optimal point, the transmittance of the MZ modulator repeats on and off at a period of 20 GHz that is twice the frequency of the applied clock voltage, outputting a CSRZ optical pulse as in FIG. 5D from the light output path 104 of FIG. 4. However, it is known that because of manufacture variation, temperature, secular change, etc., the LN-MZ modulator like this inevitably leads to a variation in the voltage of the optimal point and exhibits temporal drift. When the bias-voltage shifts from the optimal point, the heights of the adjacent pulses become unequal, as in FIG. 5C, and accordingly large waveform distortion and degradation in the transmission characteristics (receiver sensitivity and transmission range) occur, which makes information transmission impossible. Because of this, it is necessary to perform automatic control so that the bias-voltage may always coincide with the optimal point.
Hereafter, the conventional automatic control of the bias-voltage will be explained. For this, the clock amplitude is made equal to V2π of the MZ modulator in FIG. 4. A part of the CSRZ optical pulse is led to a low-speed photodetector 108 with the use of an optical coupler 106, temporally averaged optical power (average optical power) is measured, and the information is outputted from an optical power signal output terminal 109. A few Hz to a few MHz at the highest are enough as a band of a photodetector required for measurement of the average optical power. FIGS. 6A, 6B, and 6C are diagrams showing a relationship between the average optical power of the CSRZ optical signal and the bias-voltage in the conventional CSRZ optical modulator. FIG. 6A shows the optical transmittance to a direct current voltage of the MZ modulator (extinction curve); FIG. 6B shows the average optical output power obtained from the low-speed photodetector 108. With a change of the bias-voltage Vb, the average optical output varies in the shape of a sinusoidal wave, and is maximized exactly when the bias-voltage coincides with a bias-voltage that minimizes the optical transmittance of FIG. 6A (optimal point of a dotted line). This is because when the clock amplitude is V2π, if the bias-voltage is set to the optimal point, the maximum and the minimum of the clock voltage exactly coincide with a maximum point of the optical transmittance of FIG. 6A, and because when the bias-voltage shifts from this point to either a larger or smaller side, a wave height of the output waveform of one side decreases and the optical power reduces. Therefore, if the maximization control for controlling the bias-voltage Vb applied to the bias control signal input terminal is so performed that the average optical power obtained from the optical power signal output terminal 109 may always be maximized, an excellent optical CSRZ pulse is always obtainable.
JP-A No. 2003-283432 describes that the above-mentioned maximization control is applicable when the amplitude of the clock signal is equal to or more than 80% of V2π (=1.6 times Vπ). FIG. 6C shows the average power of the CSRZ optical pulse when the clock amplitude is reduced to be lower than V2π. As the clock amplitude becomes smaller, the amplitude of the average power becomes smaller. At an amplitude of about 70% of V2π, its phase is reversed, and conversely, when the clock amplitude becomes about 40% of V2π, the average optical power is minimized at the optimal bias-voltage. Because of this, JP-A No. 2003-283432 describes an example where, when the clock amplitude becomes small, minimization control is applied.